1. Field of the Invention
The present invention relates generally to the field of display devices. In particular, the present invention relates to an optical system and method for projecting an image onto a display device. More specifically, the present invention relates to an optical system and method for projecting an image onto a poly-planar optical display (POD) device.
2. Description of the Background
In the field of image projection of a rectilinear object to a proportionately enlarged or reduced rectilinear image (as represented by conventional photographic enlargers and slide projectors), the entire image is projected typically upon a single plane (e.g., in the enlarger, to the photographic paper; and from the slide projector, to the screen). A more difficult task arises when an image must be projected into a display device having two separate image surfaces for the vertical and horizontal components of the image, each of which requires independent magnification and focus of the vertical and of the horizontal image components. The problem is further complicated when one of the image surfaces is tilted with respect to the projection axis, the tilt being so significant that conventional image focus will not be sustained along the full image surfaces. The two disparate image surfaces must be illuminated in such a manner that the vertical and the horizontal image components maintain independent focus along their respective tilted surfaces. Further, since projected images generally expand (or enlarge) over progressively greater projected field distances, tilted image surfaces are also subject to xe2x80x9ckeystoningxe2x80x9d, whereby one dimension (say, the horizontal xe2x80x9cwidthxe2x80x9d) is enlarged progressively more as viewed from the xe2x80x9ctopxe2x80x9d or the xe2x80x9cbottomxe2x80x9d of the image.
An example of a device which requires such image handling is represented in U.S. Pat. No. 5,381,502 entitled, xe2x80x9cFlat or Curved Thin Optical Display Panelxe2x80x9d. FIG. 1 illustrates the type of panel construction described in the ""502 patent. The panel comprises a stack of thin planar waveguide-like transparent lamina 111 each of typical thickness t. When the stack is cut at an acute angle S, each lamination exhibits a height h at the display surface such that h=t sec S. Thus, with S measuring typically about 70.degree., h is significantly larger than t. Also, the full display height H is larger than the base thickness T by the same factor, sec S.
The device of the ""502 patent is called a xe2x80x9cpolyplanar optic displayxe2x80x9d (POD). The rightmost portion of the POD is represented primarily in FIG. 1 as an isometric view. The full width W is typically wider than its display height H. The portion which is detailed serves to describe the operation of the POD and is useful in understanding its relationship to the present invention. Each lamination (of thickness t) of the panel is a transparent sheet (glass or plastic) of nominal optical index of refraction n1, separated by relatively thin coatings (cladding) having an index of refraction n2, where n1 greater than n2. The cladding may be, for example, an opaque epoxy. Light entering the laminations at the base (input surface) is separated into sheets and is confined to its respective sheets by total internal reflection at the interfaces, i.e. at the cladding. The cladding serves to waveguide the image from one surface of the display to the other surface while absorbing scattered light that would otherwise reduce the contrast of the display. Thus, light focused at the base will retain xe2x80x9cverticalxe2x80x9d resolution elements of thickness t (in the xe2x80x9cTxe2x80x9d-direction) throughout its propagation xe2x80x9cupwardxe2x80x9d to the display surface, where each thickness t is displayed as a corresponding resolvable height h. In the width W direction, however, there is no confinement of the input illumination, and each sheet propagates its respective slice (in the width direction) as would a continuous transparent medium. This requires that the horizontal image components be focused over varying distances corresponding to the tipped viewing surface (output surface). While the vertical component of the projected image must focus near the base, the horizontal information must focus near the sloping plane of the display surface (output surface); those components at the xe2x80x9cbottomxe2x80x9d of the display focusing close to the base, and those higher focusing at progressively greater distances to represent image elements approaching the top of the display. Also, while propagating through the lamina, the horizontal components expand progressively as an extension to the expanding illuminating field. Unless corrected, this generates keystoning, whereby (in this example) the top of the displayed image becomes wider than that at the bottom.
U.S. Pat. No. 6,012,816 entitled, xe2x80x9cOptical Projection Apparatus and Methodxe2x80x9d describes an attempt to solve the aforementioned problems by providing image projection that can be used in conjunction with a POD type of display panel. However, the optical system of the ""816 patent is difficult to adjust and/or fine-tune to obtain optimal image accuracy. Depending on how it is configured, the optical system may also be considered inefficient in projecting the image to a tilted input surface such that light corresponding to the image does not enter at least one of the waveguides at an entrance angle which is substantially parallel to a plane corresponding to one of the waveguides. When light does not enter the waveguides at an entrance angle which is substantially parallel to a plane corresponding to one of the waveguides, light loss may occur from the many discreet bounces that the light undergoes in the waveguides at the cladding. Additionally, the ""816 patent is limited in design to optical systems requiring simple cylindrical lenses. The present invention also uses cylindrical lenses, but, in contrast to the ""816 patent, uses cylindrical lenses in a more elegant fashion.
It is among the objects of the present invention to solve image handling problems of the type described above and also to provide image projection that can be used in conjunction with a POD type of display panel.
The present invention is directed to an optical system for projecting an image onto an input surface of a display to be observed by an observer at an output surface of the display. The optical system comprises an image source, an imaging element, and a telescope. The optical system may also include a telecentric element and/or path-reduction prism. The display may comprise, for example, a plurality of stacked optical waveguides, each having a first end and a second end, wherein the input surface is defined by the plurality of first ends, and wherein the output surface is defined by the plurality of second ends. The optical system is preferably used when the tilt angle of the input surface is different than the tilt angle of the output surface. The optical system provides for a magnification to the input surface to be different than a magnification to the output surface. In a preferred embodiment, the tilt angle of the input surface is non-perpendicular to a plane corresponding to one of the waveguides, and wherein the prism directs the image onto the input surface at an angle which is non-perpendicular to the input surface such that light corresponding to the image enters at least one of the waveguides at an entrance angle which is substantially parallel to the plane corresponding to one of the waveguides.
The present invention solves problems experienced in the prior art by providing an optical system having a reduced optical path that produces an accurate image on a tilted input surface of a display relative to the image path, and which does not suffer from improperly focused images and image distortions which yield false aspect ratios of the original image and inconsistent, linear point-to-point mapping of the original image to the displayed image. The present invention also retains the advantages which correspond to a stacked waveguide panel, such as improved contrast and minimized depth.
Those and other advantages and benefits of the present invention will become apparent from the detailed description of the invention provided below.